”Drinking water to shed weight is a waste of time”, is the headline in the Daily Mail. Research has found that drinking water, in particular the recommended eight glasses a day...

”Drinking water to shed weight is a waste of time”, is the headline in the Daily Mail. Research has found that drinking water, in particular the recommended eight glasses a day, to keep slim is a waste of time as it does not help you shed the pounds. People could be better off “eating foods rich in water, such as fruit, vegetables, rice, soups and casseroles”, the newspaper says.

The study involved more than 1,000 young women whose weight and waist measurement was compared with the amount of water that they drank each day. The study found no link between body size and fluids drunk, but there was a link with the water content of foods eaten. However, the study took place in a group of healthy Japanese women of normal weight who were not dieting. Diet and exercise are always going to have the greatest effect on body weight and adequate intake of water and other fluids throughout the day is necessary for the health of the body.

Where did the story come from?

Kentaro Murakami and colleagues of University of Tokyo, Wayo Women’s University and Kagawa Nutrition University, Japan, carried out this research. No sources of funding were reported for this study. The study was published in the peer-reviewed medical journal: Nutrition.

What kind of scientific study was this?

This was a cross-sectional study examining the association between water intake (consumed in both beverages and in foods) and waist circumference and BMI in a sample of women. The researchers recruited 1,176 female dietetic students (aged 18 to 22) from academic institutions across Japan. They excluded those with very low or very high daily energy intakes, those currently receiving dietary counselling and those with diabetes, high blood pressure or cardiovascular disease, leaving a total of 1,136 women.

The researchers looked at dietary intake over a month using a questionnaire that assessed the consumption of different foods, alcoholic drinks and dietary supplements and also looked at the cooking methods. The average daily intake for a total of 150 food and drink items was estimated, and then the overall water intake was worked out by adding together all beverages and the water content of different foods. Height and weight were measured and the BMI calculated, and waist circumference was also measured. The participants also completed a lifestyle questionnaire that looked at demographic details, including smoking, physical activity and weight loss aims. Statistical methods were used to look at the relationship between BMI and waist circumference and the water consumed.

What were the results of the study?

Average body weight in the sample was 53.6kg and BMI 21.3 and average waist circumference 72.9cm. Only 8% of the group were overweight or obese. After adjustment for possible confounding factors, there were no significant associations between water intake through beverages and BMI or waist circumference. Increased water content of foods consumed, however, did show significant correlations, with increase in water content of foods being associated with slight decreases in waist circumference and BMI.

They found that as intake of water increased, the ratio of energy intake to energy expenditure decreased, that is, people who were taking in more water were also using up more energy through activity than they were taking in through food.

Looking at individual dietary factors, increased water intake through drinks was associated with increased fibre intake, decreased fat intake and decreased total energy intake during the day. Increased water intake in foods was associated with foods higher in protein, carbohydrate and fibre, and lower in fat.

What interpretations did the researchers draw from these results?

The researchers conclude that increased intake of water through foods is associated with decreased waist circumference and BMI, but this association is not seen with increased water through beverages.

What does the NHS Knowledge Service make of this study?

This study found associations between increased water content in foods eaten and BMI, which seems entirely plausible, as the higher water content is often associated with lower calorie and fat containing foods. However, there are limited conclusions that can be made from this study alone.

The cross-sectional associations examined do not provide information about the effect of water upon body size or weight loss. Taking two groups of people with different water intakes and then following them over time to see how weight loss differed, would provide more meaningful information.

This study is of healthy, normal weight young women and most of them were not trying to lose weight.

All confounding variables that may have affected body size have not been examined. In particular, as the authors acknowledge, physical activity was assessed “relatively roughly”.

These were also all Japanese women and the Japanese diet differs quite considerably from the average Western diet. They were also all dietetic students and therefore cannot be considered representative of the rest of the Japanese population, or of men.

Dietary and activity questionnaires always involve some degree of reporting error, as the participants have to remember activity levels and quantities of food and drink taken over a period of time. Also, this study only assessed over a one-month period, which may not be representative of the longer-term pattern. Of note, the dietary questionnaire has only been validated for use among older people, and not this younger population group.

Diet and exercise are always going to have the greatest effect on body weight. If someone was drinking a lot of water but still maintaining a high-fat and sugar diet and taking little activity, then it would not be surprising that they did not lose weight. This is a different case from someone who chooses to drink fluids instead of snacking.

Whatever its effects upon weight, adequate intake of water and other fluids throughout the day is necessary for the health of the body.

Sir Muir Gray adds...

It is less food and more walking that is the answer; water is irrelevant.